The 5-HT2B receptor, in athe gut, has generally been assumed to be a smooth muscle receptor since its discovery in the fundus (rumen) of the rat stomach. We have recently observed, however, that mRNA encoding the 5-HT2B receptor is also expressed in the intestine; moreover, we have cloned the 5-HT2B receptor from isolated ganglia of the guinea pig myenteric plexus, as well as from the small intestine of the rat and mouse mRNA encoding the receptor is selectively missing from the aganglionic terminal colon of ls/ls mice, suggesting that intestinal 5-HT2B expression, in contrast to that of the stomach, may be in neurons. In situ hybridization, confirmed the presence of mRNA encoding the 5-HT2B receptor in enteric neurons. In situ hybridization, confirmed the presence of mRNA encoding the 5-HT2B receptor in enteric neurons. Although such cells are relatively rare in adults, they are abundant in fetal mice. 5-HT2B mRNA appears in enteric ganglia at E15 and is abundant in virtually every myenteric ganglion at E16. At this time, ganglia contain proliferating neural precursors that coexist with mature serotonergic neurons (among the first enteric neurons to be born). E15-16 is also when 5-HT-secreting EC cells appear. The timing of 5-HT2B expression, its developmental regulation in enteric ganglia, and its ability to induce proliferation when expressed by transfected cells, suggested that stimulation of the 5-HT2B receptor might enable 5-HT to influence the enteric neuronal development. This hypothesis was strongly supported by the observation that 5-HT (1-10 muM) promotes the development of neurons in vitro, when added to dissociated cells from the E16 fetal mouse intestine. This effect of 5-HT was abolished by methysergide, which is an antagonist at 5-HT2B receptors. We now propose to t est the hypothesis that 5-HT, released either by the early-developing serotonergic neurons or EC cells, exerts a growth factor-like effect on 5-HT2B receptors that is physiologically important in the development of the enteric nervous system (ENS). We will test this hypothesis by: (ki) identifying the cells in developing gut that express the 5-HT2B receptor, (whether they are of neural crest origin and, if so their lineage); (ii) identifying the cells upon which 5-HT acts to promote enteric neuronal development and whether that effect is due to enhanced proliferation, differentiation and/or survival; (iii) determining whether the early expression and developmental regulation of enteric 5-HT2B receptors are true also of other 5-HT receptor subtypes; (iv) identifying the properties of the ENS that are abnormal in mice lacking the 5-HT2B receptor. We will also determine whether the shorter splice variant of the 5-HT2B receptor that we have found is capable of binding ligand and transducing responses.